SSL: optimized rotation of session ticket keys. Instead of syncing keys with shared memory on each ticket operation, the code now does this only when the worker is going to change expiration of the current key, or going to switch to a new key: that is, usually at most once per second. To do so without races, the code maintains 3 keys: current, previous, and next. If a worker will switch to the next key earlier, other workers will still be able to decrypt new tickets, since they will be encrypted with the next key.

--- a/src/event/ngx_event_openssl.h
+++ b/src/event/ngx_event_openssl.h
@@ -160,7 +160,7 @@ typedef struct {
     ngx_rbtree_t                session_rbtree;
     ngx_rbtree_node_t           sentinel;
     ngx_queue_t                 expire_queue;
-    ngx_ssl_ticket_key_t        ticket_keys[2];
+    ngx_ssl_ticket_key_t        ticket_keys[3];
     time_t                      fail_time;
 } ngx_ssl_session_cache_t;